This invention relates to water purification and particularly to a system for cleaning water from mining operations.
Mining activities in the western United States have created thousands of Acid Mine Drainage (AMD) generation sites. The contamination level of each site differs as to the number of metal contaminants and the acid content of the waste stream.
These contaminants are usually a mixture of heavy and light metals such as iron. copper zinc, magnesium, manganese, aluminum, cadmium, nickel and lead. the acid and sulfate levels vary greatly.
U.S. Pat. No. 5,698,107 to Wurzburger et al discloses a process that is capable of treating heavily contaminated acid mine water. The high metal ion content and acid levels of such waste streams are highly electrically conductive and therefor easily treated by this process.
Iron Mountain Mine near Redding Calif., and the Leviathan Mine in Alpine County, Calif.
These three sites are unique in that they contained moderate to very high concentrations of dissolved metals and sulfates.
At the time that the treatibility studies were being done, AMD from Iron Mountain Mine had a soluble sulfate level of 10,800 parts per million, AMD from Berkeley Pit had a soluble sulfate level of 6930 parts per million which are rising at almost 10% per year and AMD from the Leviathon mine had 5900 part s per million.
State of the art AMD treatment technology use a variation of a liming process to raise the pH causing most of the metals to precipitate and to reduce sulfates. Unfortunately, these processes precipitate tons of hazardous waste which is a mixture of metal hydroxides and calcium sulfate. The handling cost required to dispose of this material is 300 to 400% of the actual water treatment cost.
U.S. Pat. No. 5,698,107 disclosed a three stage process in which lime was used only in the second stage to create a metal free anhydrous calcium sulfate cake. Although this material is safe for on-site storage and could have value as a soil conditioner, the process is an expensive use of valuable sulfate compounds.
Separation of sludges of precipitated matter is an inherent problem in the present art of water treatment. There are many technologies in which filtration systems have been devised to separate solid matter from liquids. In one type of filter constuction, developed for the separation of colloidal suspensions, the raw liquid flows tangent to an absorbing (cloth) medium. The absorbing medium absorbs the permeate from the stream and is drawn off the side of the absorbing (filtering) medium opposite the stream thereby continuously increasing the concentration of the colloidal suspension in the stream.
This method of filtration is referred to herein as xe2x80x9ctangential filtrationxe2x80x9d. A system incorporating this method of filtration adapted for filtering colloidal suspensions using a filter element having a tosion resistanct fiber structure is diclosed in U.S. Pat. No. 5,725,767 to Culkin which is hereby incorporated by reference into this specification.
It is an object of this invention to provide a method for processing AMD initially having a low concentration of metal (such as the discharge from the drainage produced from the process disclosed in U.S. Pat. No. 5,698,107) to virtually non-detectable concentrations.
It is another object of this invention to apply a novel technology involving the use of membranes to remove any metal and sulfate contaminants remaining in water discharged from the disclosed processes of the prior art.
As an additional benefit of the invention, it is an object to produce a sulfate rich side stream of sufficiently high concentration to have commercial value.
It is contemplated to use a tangential filtration system in a novel application wherein the stream is directed against a membrane that will pass only free water molecules thereby preventing unprecipitated cations and most all anions from entering the discharge stream. This aspect of the invention is particularly valuable in certain situations where safety must be considered.
It is another object of this invention to generate a high pH effluent from part of the waste stream containing sulfate ladened water that is used as the source of hydroxide in the demetallized stage of the invention.
It is another object to add ammonium hydroxide to a remaining portion of the sulfate stream to generate ammonium sulfate fertilizer.
This invention is directed towards a process for mixing precipitating agents_containing a high pH effluent with AMD to generate pure water, aqueous ammonia and cakes of calcium sulfate, ammonium sulfate. All of these compounds are intended for commercial distribution.
The apparatus for performing the method of this invention is best described as comprising three sections, I, II, III. In the context of this specification, a xe2x80x9chigh pH effluentxe2x80x9d is understood to contain OH and a cation (K+) in a ratio greater than 400/20 by weight. xe2x80x9cSulfate ladened waterxe2x80x9d is understood to mean water containing more than 1000 ppm. The term, xe2x80x9cpurified waterxe2x80x9d, is understood to mean water having less than 100 ppm impurity content. The term, xe2x80x9cdemetallized waterxe2x80x9d is understood to mean water containing less than 1 ppm metal ions.
Section I
Untreated AMD enters section I to which is added a precipitating agent of Mg(OH)2 and high pH effluent from section II. Precipitation of metal hydroxides ensue which is separated as a cake leavng a stream of xe2x80x9cdemetallized waterxe2x80x9d It is a feature of this invention that xe2x80x9ctangential filtrationxe2x80x9d is used to separate purified water (as defined above) from sulfate ladened water (defined above).
This is a wholly unexpected result in view of the fact that xe2x80x9ctangential filtrationxe2x80x9d has been used prior to this disclosure to separate colloidal suspensions from water and its use to filter out sulfate ions has not been reported heretofore.
Section I produces purified water and sulfate laden water.
Section II
One portion of sulfate ladened water from section I is mixed with Ca(OH)2 and enters section II where there is produced a calcium sulfate cake and high pH effluent. The cake is conveyed out of section II preferably for commercial distribution. The high pH effluent is conveyed out of section II back to section I where it is used as a prcipitating agent as discussed above.
Section III
The remaining portion of sulfate ladened water from section I is mixed with ammonia to produce anhydrous ammonia sulfate cake and aqueous ammonia suitable for commercial distribution.
Various features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings.